1. Field of the Invention
This invention relates to an image formation apparatus such as a copying apparatus having various warning and display functions.
2. Description of the Prior Art
Particularly when a thermistor is brought into contact with a heat source to detect the temperature of the latter, the thermistor may be damaged by the rotation of the heat source or a lead wire supporting the thermistor may be broken. To avoid this, a method of protecting the surface of the thermistor by a heat-resistant tape or a method of bringing the thermistor into contact with the heat surface through a metal of springy property has heretofore been adopted. Recently, however, to reduce the actual space occupied by the temperature control, there is a tendency to miniaturize the thermistor itself and accordingly the lead wire of the thermistor and so, the danger of disconnecting or damaging the thermistor is increasing without any measure for preventing such disconnection or damage being proposed.
If temperature control is effected by a disconnected thermistor, the thermistor could not distinguish between the disconnected condition thereof and the low temperature condition of the detecting portion and would permit electric power to continue to be supplied to the heat source, thus causing abnormal temperature rise of the heat source. Such abnormal temperature in turn would cause deformation of a fixing roller as the movable part of the heat source or the heat-resistant resin on the surface of the fixing roller which would prevent smooth rotation of the fixing roller and accordingly normal fixation.
FIG. 17 of the accompanying drawings shows a circuit which may judge a difference in resistance value of the thermistor and detect disconnection thereof. This comprises a well-known bridge type temperature detecting circuit having added thereto a disconnection detecting bridge consisting of R6, R7, R3 and TH1, and is designed such that disconnection of the thermistor TH1 is detected by an operational amplifier Q2 to turn off a transistor Q3 and thereby turn off the power supply to a heater H1.
In this case, if electric power is not supplied to the entire apparatus including the heater for a long time and moreover the apparatus is left at a low temperature, then the thermistor TH1 would present a high resistance of the order of 10M.OMEGA.. The distinction between such high resistance and the high resistance resulting from a disconnection could only be realized either by using as the operational amplifier an operational amplifier of small offset and high input impedance or by increasing the voltage Vcc to detect a minute resistance variation in the form of a great potential difference. The former method not only requires the use of an expensive operational amplifier but also requires the provision of a counter-measure for noise and accordingly an extra device because a minute signal indicative of a disconnection is detected while being distinguished from the noise. In the latter method, the thermistor may be damaged by the thermistor current flowing when the temperature has become high and the resistance value of the thermistor has become low, and accordingly, the resistance value R3 must be determined so that the electric power of the thermistor does not exceed a rated loss power. However, such determination is very difficult and moreover, one more kind of Vcc voltage would be required and the power source would become complicated.
It is usual with copying apparatus that start of the copying is prevented until the fixing device reaches a predetermined temperature, but thereafter during the temperature control, the copying operation and the temperature control are rendered independent of each other. Accordingly, the thermistor may be disconnected to thereby cause abnormal temperature rise which may damage the copying parts as previously described.
Also, in a compact copying apparatus, elements such as semiconductor circuit elements which may be damaged by excessive heat are arranged in a crowded fashion and therefore, temperature rise in the interior of the apparatus resulting from disconnecting or over-heating of the thermistor would lead to destruction of the machine.
Particularly, in a transfer type copying apparatus having a small-sized photosensitive drum, the drum is disposed near the fixing device and thus, disconnection or over-heating of the thermistor would hasten deterioration of the photosensitive medium.
Also, copying apparatus are known which have the function of detecting and displaying whether or not the fixing device has reached the fixing temperature and the functions of detecting and displaying a deficiency of developer such as toner, deficiency of copy mediums such as transfer paper and jam of copy mediums.
In such apparatus, a display device is provided for each warning function and therefore, if it is desired to monitor the interior of the apparatus in every respect, a great number of warning display devices would be required and this, coupled with the necessity of providing a greater number of operating keys, would complicate the operating portion. Also, in the past, each display device has a single function of only effecting its exclusive warning display or only stopping the operation of the apparatus simultaneously with the warning and thus, it has been the case that the apparatus is stopped with inordinate frequency or that the smoothness with which the copying is restarted is hampered.